Tomato Mac 'n' Cheese

2 cups uncooked macaroni
1 (16 ounce) can tomato sauce
3 tablespoons chopped onions
2 1/2 cups finely shredded reduced fat sharp Cheddar cheese
Tomato slices (optional)

Cook macaroni; drain, and set aside.

Heat oven to 350 degrees F. Lightly spray a 2-quart casserole with nonstick spray.

In a medium bowl, stir together onions and tomato sauce. Spread a thin layer of the tomato sauce mixture in the bottom of the prepared dish. Place 1/3 of the macaroni in the dish, then top with 1/3 of the remaining sauce mixture and 1/3 of the cheese. Repeat layering of macaroni, sauce and cheese two more times. Bake for 30 to 35 minutes or until bubbly and heated through. Garnish with tomato slices, if desired.